
Feasible Communication Range. Generally, WPT has a much shorter range than data transmission as indicated in [1]}. Thus, the feasible communication range of Ue-WPIoT is essentially limited by the achievable WET range. The communication range constraint may limit the wide application of Ue-WPIoT in different scenarios.

Energy-efficiency optimization. UAVs are also suffering from the limited energy to support both flight propulsion and Ue-WPIoT communications. The energy consumption of the whole flight occupies the major proportion of the entire energy consumption. Even though UAVs can harvest energy from the ambience (e.g., solar panels), the ambience energy sources being susceptible to environment fluctuations cannot be the stable energy sources for UAVs. Thus, it is a critical issue to optimize the energy-efficiency with consideration of flight time, multi-access and trajectory design.

Flying Trajectory Design. In Ue-WPIoT, a UAV is expected to conduct tasks to visit multiple IoT nodes along a given trajectory. It is challenging to design the optimal trajectory, which requires the joint consideration of multiple factors, such as multi-node access, multi-task scheduling and the total flight time.

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